A wide variety of medical treatments can be facilitated by occluding body lumens or cavities such as arteries, veins, aneurysms, vascular malformations, arteriovenous fistulas, fallopian tubes, ureters, cystic ducts, or vas deferens. Endovascular occlusion approaches typically involve placing surgical implements or implants within the vasculature of the human body, for example, typically via a catheter (see e.g., U.S. Pat. Nos. 4,884,575 and 4,739,768, both to Engelson), either to block the flow of blood through a vessel making up that portion of the vasculature through the formation of an embolus or to form such an embolus within an aneurysm stemming from the vessel.
Occlusion of vascular structures by endovascular catheters has been realized through the use of detachable balloons, injectable glue, embolic coils, and injectable particles. Detachable balloons are of such a nature that they can only be practically used in large vessels. The use of injectable glue is limited by the difficulty of controllable delivery to the desired occlusion site. The use of injectable particles suffers from their relative invisibility in fluoroscopy and the difficulty in controlling their ultimate disposition at the desired occlusion site.
A highly desirable embolism-forming device that may be introduced into an aneurysm using endovascular placement procedures, is found in U.S. Pat. No. 4,994,069, to Ritchart et al. The device, typically a platinum/tungsten alloy coil having a very small diameter, may be introduced into an aneurysm through a catheter such as those described in Engelson above. These coils are often made of wire having a diameter of 2-6 mils. The coil diameter may be 10-30 mils. These soft, flexible coils may be of any length desirable and appropriate for the site to be occluded. For instance, the coils may be used to fill a berry aneurysm. Within a short period of time after the filling of the aneurysm with the embolic device, a thrombus forms in the aneurysm and is shortly thereafter complemented with a collagenous material which significantly lessens the potential for aneurysm rupture.
Coils such as seen in Ritchart et al. may be delivered to the vasculature site in a variety of ways including, e.g., mechanically detaching them from the delivery device as is shown in U.S. Pat. No. 5,250,071, to Palermo or by electrolytic detachment as is shown in Guglielmi et al. (U.S. Pat. No. 5,122,136), discussed below.
U.S. Pat. No. 5,250,071 to Palermo discloses a coil having interlocking clasps with a delivery device, the clasps being secured together by a control wire and released upon withdrawal of the control wire. Another mechanically detachable coil is described in U.S. Pat. No. 5,261,916 to Engelson which discloses a pusher-vasoocclusive coil assembly having a ball on the proximal end of the coil interlockingly engaged with a keyway at the distal end of the pusher. The ball is biased on the coil to be disengaged with the keyway, and is coupled with the keyway by radially enclosing the assembly within a microcatheter. Withdrawal of the microcatheter allows the bias of the ball to disengage the coupling.
U.S. Pat. No. 5,122,136 to Guglielmi et al. discloses an electrolytically detachable coil. The coil is attached to the distal end of an insertion device by a sacrificial joint or link that is electrolytically dissolvable upon application of a small DC current. The return electrode is typically placed on the patient's skin.
U.S. Pat. No. 5,423,829 describes a variation of the Guglielmi detachable coil using an improved sacrificial link between the guidewire and the coil. The size of the sacrificial link is limited to allow more precise placement of the embolic device and facilitate quick detachment. The focussed electrolysis found at the sacrificial site reduces the overall possibility of occurrence of multiple electrolysis sites and liberation of large particles from those sites.
The circuit involved in the electrolytic coil detachment arrangements discussed above generally includes a power source having its positive terminal coupled to the sacrificial link via a guidewire, for example. More specifically, a positive electric current of approximately 0.01 to 2 milliamps is applied to the guidewire which is coupled to the sacrificial link that is intended to undergo electrolytic disintegration and which initially couples the implant (e.g., the vasoocclusion device) to the guidewire. The negative terminal of the power source is typically coupled to an electrode that is placed over and in contact with the patient's skin.
Another return electrode or cathode arrangement is disclosed in U.S. Pat. No. 5,364,295 to Guglielmi et al. In that arrangement, the microcatheter is supplied with an end electrode. More specifically, the electrode extends distally from the microcatheter and is coupled to an electrical conductor or wire disposed along the length of the microcatheter. The wire is ultimately led back to the negative terminal of the power source so that the electrode (ring electrode) is used as the cathode during electrothrombosis instead of an exterior skin electrode.
According to the '295 patent, the electrical currents and electrical current paths which are set up during electrothrombosis formation using the above-described catheter-electrode arrangement are local to the site of application, which allows even smaller currents and voltages to be used to initiate electrothrombosis.
Another embolic device is the liquid coil, which has little or no inherent secondary shape. U.S. Pat. No. 5,690,666 discloses a coil having little or no shape after introduction into the vascular space.
In addition to delivering embolic coils, other well known endoluminal occlusion techniques have involved passing direct current (DC) or alternating current (AC) through tissue to create an occlusive tissue response. Such techniques generally require an occlusion electrode, usually disposed on an endoluminal device within the target lumen or cavity, and a reference electrode, usually comprising a patch on the skin. A DC or AC power source coupled to the electrodes applies direct or oscillating current, respectively, between the two electrodes and through the tissue.
Publications describing the use of DC electrocoagulation for occlusion include: Thompson et al., "Vessel Occlusion with Transcatheter Electrocoagulation: Initial Clinical Experience," Diagnostic Radiology at 335 (November 1979); Thompson et al., in "Transcatheter Electrocoagulation: A Therapeutic Angiographic Technique for Vessel Occlusion," Investigative Radiology at 146 (March-April 1977); Phillips, "Transcatheter Electrocoagulation of Blood Vessels," Investigative Radiology at 295 (September-October 1973); and Phillips et al., "Experimental Closure of Arteriovenous Fistula by Transcatheter Electrocoagulation," Diagnostic Radiology 115:319.
As described in the above publications, the occlusion electrode is generally used as the anode and a constant current supply is usually used. The DC current is generally delivered over extended periods of time to achieve coagulation which occludes a lumen. Delivery of 10-15 mA of direct current for a time period ranging from 6-80 minutes has generally been required for DC electrocoagulation. Observed negative implications of this level of direct current over the time required for occlusion have included burns at the electrode sites, electrode fragmentation into patient tissues, and pain requiring administration of Morphine, Demorol, or other pain killers. It is believed that electrothrombosis from DC currents is in part due to attraction of negatively charged platelets to the positively charged occlusion electrode (anode), and in part to a like attraction of platelets to thermally injured and positively charged wall components.
In the case of AC currents used for occlusion, much higher peak currents than those disclosed in DC uses have been safely used to create occlusions. For example, Gold et al., in "Transarterial Electro-coagulation Therapy of a Pseudoaneurysm in the Head of the Pancreas," American Journal of Roentgenology (1975) 125(2) :422 disclosed that 500 mA of current in an AC electrocoagulation device was delivered safely in preliminary studies. Nevertheless, radio-frequencies (RF) are generally used for AC occlusion, as lower frequencies have been observed to cause fibrillation.
An example of the tissue response to RF oscillating currents is described by Becker et al. in "Long-Term Occlusion of the Porcine Cystic Duct by Means of Endoluminal Radio-Frequency Electrocoagulation," Radiology (1988) 167:63-68. Becker et al. disclose using RF power with bipolar occlusion electrodes to occlude the cystic ducts in pigs. Maximum duration to achieve occlusion was 24 seconds, peak current flow levels ranged from 100 to 425 mA, and all test ducts were occluded with an acute narrowing of the ducts observed. Becker et al. also observed an inherent limitation to their RF technique: adherence of the occlusion electrodes to the tissue at the occlusion site and damage upon subsequent withdrawal of the device from the occlusion.
Another example of the tissue response to "high frequency" electro-occlusion is disclosed in U.S. Pat. No. 4,057,063 to Gieles et al. Gieles et al. disclose coagulating and desiccating the fallopian tubes for sterilization of human females using a "high frequency" generator that is coupled to an electrode fixed to the end of a catheter. Gieles discloses that a series of high frequency pulses causes coagulation, desiccation, and ultimately carbonization of the patient tissue in the fallopian tube.
One mechanism for monitoring the progression of occlusion, disclosed by Gieles et al. (supra), signals a user of an RF occlusion device when the procedure is complete. Upon applying high frequency energy to the target fallopian tube tissue, lamps are energized and extinguished to signal completion of occlusion based upon monitored changes in the rms values of current and voltage delivered by the generator. The RF power generator is then shut down manually by the user when sign al led to do so by the lamps.
U.S. Pat. No. 4,907,589 to Cosman discloses an over-temperature control apparatus for an RF therapeutic heating device. The apparatus provides a combined manual and automatic temperature control of heating of biological tissue by an electrode.
There is a need to provide apparatus for accurately detecting electro-occlusion and an apparatus for automatically terminating the power output in an electro-occlusion device in response to detecting occlusion.